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Snow
Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. A snowflake is an aggregate of ice crystals that forms while falling in and below a cloud. Snowflakes are typically hexagonally symmetrical. Geometry Large, well formed snowflakes are relatively flat and have six approximately identical arms, so that the snowflake nearly has the same 6-fold dihedral symmetry as a hexagon or hexagram. This symmetry arises from the hexagonal crystal structure of ordinary ice. However, the exact shape of the snowflake is determined by the temperature and humidity at which it forms. Rarely, at a temperature of around -2 °C (28 °F), snowflakes can form in threefold symmetry - triangular snowflakes. Snowflakes are not perfectly symmetric however. The most common snowflakes are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing. Snowflakes can come in many different forms, including columns, needles, bricks and plates (with and without "dendrites" - the "arms" of some snowflakes). These different forms arise out of different temperatures and water saturation - among other conditions. Six petaled ice flowers grow in air between 0 °C (32 °F) and -3 °C (27 °F). The vapor droplets solidify around a dust particle. Between temperatures of -1 °C (30 °F) and -3 °C (27 °F), the snowflake will be in the form of a dendrite or a plate or the six petaled ice flower. As temperatures get colder, between -5 °C (23 °F) and -10 °C (14 °F), the crystals will form in needles or hollow columns or prisms. When the temperature becomes even colder (from -10 °C to -22 °C) the ice flowers are formed again, and at temperatures below -22 °C, the vapors will turn into prisms again. If a crystal has started forming at around -5 °C, and is then exposed to warmer or colder temperatures, a capped column may be formed which consists of a column-like design capped with a dendrite or plate-like design on each end of the column. At even colder temperatures, the snowflake design returns to the more common dendrite and plate. At temperatures approaching -20 °C, sectored plates are formed which appears as a dendrite, with each dendrite appearing flattened, like the design of a snowflake plate. There are, broadly, two possible explanations for the symmetry of snowflakes. First, there could be communication or information transfer between the arms, such that growth in each arm affects the growth in each other arm. Surface tension or phonons are among the ways that such communication could occur. The other explanation, which appears to be the prevalent view, is that the arms of a snowflake grow independently in an environment that is believed to be rapidly varying in temperature, humidity and other atmospheric conditions. This environment is believed to be relatively spatially homogeneous on the scale of a single flake, leading to the arms growing to a high level of visual similarity by responding in identical ways to identical conditions, much in the same way that unrelated trees respond to environmental changes by growing near-identical sets of tree rings. The difference in the environment in scales larger than a snowflake leads to the observed lack of correlation between the shapes of different snowflakes. The sixfold symmetry happens because of the basic hexagonal crystalline structure from which the snowflake grows. The exact reason for the threefold symmetry of triangular snowflakes is still a mystery although trigonal symmetry is a subsymmetry of hexagonal. There is a widely held belief that no two snowflakes are alike. Strictly speaking, it is extremely unlikely for any two macroscopic objects in the universe to contain an identical molecular structure; but there are, nonetheless, no known scientific laws that prevent it. In a more pragmatic sense, it's more likely—albeit not much more—that two snowflakes are visually identical if their environments were similar enough, either because they grew very near one another, or simply by chance. The American Meteorological Society has reported that matching snow crystals were discovered in Wisconsin in 1988 by Nancy Knight of the National Center for Atmospheric Research. The crystals were not flakes in the usual sense but rather hollow hexagonal prisms. Types of snow Falling Snow ;Blizzard : A long-lasting snow storm with intense snowfall and usually high winds. Particularly severe storms can create whiteout conditions where visibility is reduced to less than 1 m. ;Columns :A class snow flakes that is shaped like a six sided column. One of the 4 classes of snow flakes. ;Dendrites :A class of snow flakes that has 6 points, making it somewhat star shaped. The classic snowflake shape. One of the 4 classes of snowflakes. ;Flurry : A period of light snow with usually little accumulation with occasional moderate snowfall. ;Freezing rain : Rain that freezes on impact with a sufficiently cold surface. This can cover trees in a uniform layer of very clear, shiny ice – a beautiful phenomenon, though excessive accumulation can break tree limbs and utility lines, causing utility failures and possible property damage. ;Graupel : Precipitation formed when freezing fog condenses on a snowflake, forming a ball of rime ice. Also known as snow pellets. ;Ground blizzard : Occurs when a strong wind drives already fallen snow to create drifts and whiteouts. ;Hail : Many-layered ice balls, ranging from "pea" sized (0.25 in, 6 mm) to "golf ball" sized (1.75 in, 43 mm), to, in rare cases, "softball" sized or greater (>4.25 in, 108 mm). ;Hailstorm : A storm of hail. If the hail is sufficiently large, it can cause damage to cars or even people. ;Lake effect snow : Produced when cold winds move across long expanses of warmer lake water, picking up water vapor which freezes and is deposited on the lake's shores. ;Needles :A class of snow flakes that are ascicular in shape (their length is much longer than their diameter, like a needle). One of the 4 classes of snow flakes. ;Rimed snow :Snow flakes that are partially or completely coated in tiny frozen water droplets called rime. Rime forms on a snow flake when it passes through a super-cooled cloud. One of the 4 classes of snow flakes. ;Sleet : In the United Kingdom, rain mixed with snow; in America, ice pellets formed when snowflakes pass through a layer of warm air, partially or completely thaw, then refreeze upon passing through sufficiently cold air during further descent. The proper term term for the American definition is "Ice Pellets." ;Snow pellets : See graupel. ;Snow squall : A brief, very intense snowstorm. ;Snow storm : A long storm of relatively heavy snow. ;Soft hail : See graupel. ;Thundersnow : A thunderstorm which produces snow as the primary form of precipitation. Snow on the ground ;Blowing snow : Snow on ground that is being moved around by wind. See ground blizzard. ;Corn : Coarse, granular wet snow. Most commonly used by skiers describing good spring snow. Corn is the result of diurnal cycle of melting and refreezing. ;Crust :A layer of snow on the surface of the snowpack that is stronger than the snow below, which may be powder snow. Depending on their thickness and resulting strength, crusts can be termed "supportable," meaning that they will support the weight of a human, "breakable," meaning that they will not, or "zipper," meaning that a skier can break and ski through the crust. Crusts often result from partial melting of the snow surface by direct sunlight or warm air followed by re-freezing. ;Depth Hoar :Faceted snow crystals, usually poorly or completely unbonded (unsintered) to adjacent crystals, creating a weak zone in the snowpack. Depth hoar forms from metamorphism of the snowpack in response to a large temperature gradient between the warmer ground beneath the snowpack and the surface. The relatively high porosity (percentage of air space), relatively warm temperature (usually near freezing point), and unbonded weak snow in this layer can allow various organisms to live in it. ;Ice : Densely packed material formed from snow that doesn't contain air bubbles. Depending on the snow accumulation rate, the air temperature, and the weight of the snow in the upper layers, it can take snow a few hours or a few decades to form into ice. ;Firn :Snow which has been lying for at least a year but which has not yet consolidated into glacier ice. It is granular. ;Packed Powder :The most common snow cover on ski slopes, consisting of powder snow that has lain on the ground long enough to become compressed, but is still loose. ;Packing snow : Snow that is at or near the melting point, so that it can easily be packed into snowballs and hurled at other people or objects. This is perfect for snow fights and other winter fun, such as making a snowman, or a snow fort. ;Penitentes : Tall blades of snow found at high altitudes. ;Powder : Freshly fallen, uncompacted snow. The density and moisture content of powder snow can vary widely; snowfall in coastal regions and areas with higher humidity is usually heavier than a similar depth of snowfall in an arid or continental region. Light, dry (low moisture content, typically 4 - 7% water content) powder snow is prized by skiers and snowboarders. It is often found in the Rocky Mountains of North America and in Niseko, Japan. ;Pukak :See Depth Hoar ;Slush : Snow which partially melts upon reaching the ground, to the point that it accumulates in puddles of partially-frozen water. ;Snowdrift : Large piles of snow which occur near walls and curbs, as the wind tends to push the snow up toward the vertical surfaces. ;Surface Hoar : Faceted, corn-flake shaped snow crystals that are a type of frost that forms on the surface of the snow pack on cold, clear, calm nights. Subsequent snow fall can bury layers of surface hoar encorporating them into the snowpack where they can form a weak layer. Sometimes referred to as hoar frost. ;Watermelon snow : A reddish/pink colored snow that smells like watermelons, and is caused by a red colored green algae called chlamydomonas nivalis ;Wind slab : A layer of relatively stiff, hard snow formed by deposition of wind blown snow on the leeward side of a ridge or other sheltered area. Wind slabs can form over weaker, softer freshly fallen powder snow creating an avalanche hazard on steep slopes.